For clarification; Your Mavic does not lose its GPS capability. Its the GPS positioning of the Remote.
You basically only lose "Follow Me" and the ability to "reset your home point" if you move to a new location after take-off and want to set a home point to a new location. For example your Mavic is following you in your car, and you are now some distance from your start point and RTH is initiated (whether manually or low battery, etc), it will return to its ORIGINAL starting point.
Personally, I've always bought the Ipads with Cellular, just makes it more simple. However, even with Cellular, if its not activated, and there is no nearby Wifi, the mini map doesn't display any map details.
I "believe" if you tether your Ipad to your Iphone you do get GPS... Someone else can verify...
My only suggestion is to buy the most powerful Ipad you can easily afford. While the Ipad
Mini 2 works, it does seem to "glitch" every now and then. It never crashes, but sometime I'd get partial screen refresh (momentarily), partial green screen, or occasional screen freeze. It is improved if you follow that guideline elsewhere in this forum where you turn off some Ipad functionality.
I have zero issues with an; Ipad
Air 2, Ipad
Mini 4, and Ipad Pro...
As others have stated 16gig is enough if you only keep a few apps on it. My Ipad
Air 2 has only the basic apps that comes with the Ipad, DJI GO, DJI GO 4, and a few drone weather apps. My Ipad
Mini 4 is a 128 gig and I have a bunch of stuff on there and it works fine. I just close all apps that I'm not using when I fly...